1 8 nd International Conference on Physical and Numerical Simulation of Materials Processing, ICPNS 16 Seattle Marriott Waterfront, Seattle, Washington, USA, October 14-17, 2016 Analysis Optimal Process Conditions and Mechanical Properties on Nanocomposites according to Structural Changes of Halloysite Nanotubes Yun-Hae. Kim 1, Antonio Norio. Nakagaito 2, and Soo-Jeong. Park 1,a 1 Department of Mechanical Engineering, Korea Maritime and Ocean University, Busan, KS012, South Korea 2 Department of Mechanical Engineering, Tokushima University, Tokushima, 770-8501, Japan a Corr. Author: blue9069@naver.com ABSTRACT In the present study, optimal dispersion conditions were developed to disperse nanocomposite containing halloysite nanotubes (HNTs) and unsaturated polyester resin (UP) using ultrasonic dispersion method. HNTs are naturally occurring layered silicate mineral and has aluminosilicate-layer in form of nanotubes. The abovementioned nanocomposite can be used as a functionally effective nanofiller which can impact mechanical strength due to restrictive matrix dislocation movements. In this regard, some studies demonstrated that nanocomposites containing HNTs can exhibit impact resistance. To note, due to presence of appreciable amount of water in HNTs, the properties of HNTs depend on temperature. Heat-treated HNTs at various temperature have different structure and mechanical/chemical properties. Therefore, it is important to understand structural changes of HNTs with change in operating temperature. To this end, in the current research, HNTs were heat-treated at various temperatures and correlation between UP and the heat-treated HNTs was studied in detail. The HNTs were treated at 2 temperatures, viz. 500 C and 700 C. The amount of HNTs used in this study was and 1 wt. %. We restricted the contents of HNTs because it will possible to show the aggregation phenomenon. Dispersion was carried out by ultrasonication. All structural changes and dispersion behavior was examined by TEM. The mechanical properties were assessed by impact tests Results showed that the nanocomposite containing 1 wt. % of 700 C heat-treated HNT was superior in high impact strength. Therefore, it can be said that the rheological property of matrix resin depended on the amount of HNTs and the heat treatment temperature. Keywords: Halloysite Nanotube (HNT), Nanocomposite, Ultrasonication, Particle Dispersion, Unsaturated Polyester Resin (UP), Impact Strength. 1. INTRODUCTION Nanocomposites are materials which take advantage of nanoparticles as reinforcement on composite materials. A nanocomposite mainly comprises a polymer matrix and a particulate filler within the scope of a hundred nanometers. The nanoparticles have spacious interfacial area compared with commonly used reinforcements. Halloysite nanotube (HNT) is a naturally occurring layered silicate mineral containing aluminosilicatelayer in form of nanotubes. HNTs are odorless, white nanoparticles with chemical formula H4Al2O9Si2 2H2O and are more economical than other nanofillers, such as carbon nanotubes. Studies showed that HNTs as additives in matrix can improve mechanical properties e.g. impact resistance and tensile strength. For example, HNTs when added to plastics are capable of mechanically strengthening polymers through restrictive matrix dislocation activity. However, it is difficult to disperse HNTs effectively into the polymer matrix in synthesis of nanocomposites due to aggregation of the HNTs. Therefore, establishing the right conditions under which homogenous dispersion of HNTs in polymer matrix can be achieved still remains a task. Currently, dispersion of HNTs is commonly achieved through mechanical dispersion, such as ball milling and ultrasonic homogenization. Especially, the ultrasonic homogenization is a typical dispersion method to achieve minimal aggregated volume in dispersions and emulsions. However, this method is more suitable for a laboratory scale than the commercial production. Besides the dispersion method, the dispersion of HNTs also depends on the operating temperature. Inside the HNTs is present appreciable amount of water between SiO4 and AlO6
2 structures. So, heat-treated HNTs at various temperatures have different structures and mechanical/chemical properties. In other words, depending upon the temperature at which HNTs have been heat-treated, the HNTs behave toward a polymer matrix and show certain inherent properties. In the current research, we investigated the structural changes and dispersion behavior of HNTs in a polymer matrix due to heat treatment of the HNTs at different temperatures. Additionally, the results of impact test were analyzed from which the optimal manufacturing conditions for HNT-reinforced nanocomposite were further established. As the final outcome, a nanocomposite material could be realized which showed much better mechanical property than the conventional composites unreinforced by nanoparticles. 2. EXPERIMENTAL WORK 2.1 Materials and Methods The materials used in this study are shown in Table 1. The nanocomposites were composed of unsaturated polyester resin (UP) matrix and HNTs reinforcement. The UP is one of the most widely used thermosetting resins. Furthermore, UP has excellent mechanical properties and simple processing. The HNTs were heat treated in Argon (SHIKOKU ASECHIREN Ltd) atmosphere at three different temperature, viz. room temperature (RT), 500 C and 700 C (Figure 1) using a heat controller (model SU02-110, CHINO Ltd.). The heat treatment process involved 3 steps as follows: first step- preheatime time: 30-40 minutes, second step- the processing time: 4 hours, third step- cooling time: 1 hour. Halloysite Nanotube (HNT) Unsaturated Polyester Resin (UP) Methyl Ethyl Ketone Peroxide (MEKP) Table 1 Description of main materials Reinforcement - Sigma-Aldrich - Japan G.K - Product No. 685445 - CAS-No. 1332-58-7 - Formula : H 4A l2o 9Si 2 2H 2O - Molecular Weight : 294,19 g/mol Matrix - SHOWA DENKO K.K. - Srider BP-1055 (Lot. KE 624PL01) - Gelation Time : 19 minutes - Optimum Hardening Time : 33 minutes - Maximum Heat-generating Temperature : 138 C - NOF CORPORATION - CAS-No. 1338-23-4 - Hardener - Specific Gravity : 1.146 g/ml at 20 C - Sigma-Aldrich Japan Cobalt - CAS-No. 61789-51-3 Naphthenate - Accelerator - Specific Gravity : 0.921 g/ml at 25 C The HNTs/UP nanocomposites manufactured were differentiated based on contents of HNTs added to UP. The amount of HNTs used were and 1 wt. % with respect to UP. Figure 1 Process diagram of heat treatment After the heating process, the heat-treated HNTs were dispersed in the UP matrix using an ultrasonic homogenizer (UH-150, SMT Co., Ltd. Japan) for 300 seconds at 45 and 60 W (Figure 2). The operating time and the volume of the mixture of UP and HNTs were maintained constant in all cases, viz. 300 sec. and 18 ml. (a) Ultrasonic homogenizer (b) Process schematic diagram Figure 2 Appearance of ultrasonic dispersion processes Six samples of such mixture were prepared through the above-mentioned ultrasonic dispersion method and listed in Table 2. Untreated HNTs Table 2 specimen types used (wt. %) 500 C heattreated 700 C heattreated 1 HNT 1 HNT 1
3 2.2 Property Evaluation The emphasis of this study was mainly laid on three aspects of the HNT/UP composite. First, the aggregation behaviour of heat-treated HNTs in the UP matrix. Seconds, bonding and aggregation states between UP and HNTs depending on the amount of HNTs. Finally, we checked whether it is possible to minimize aggregation by controlling the output power of the ultrasonic homogenizer. We assumed that higher the heat treatment temperature, larger will be the change in the layer structure of the HNTs which in extreme limit can be even broken. The basic structure and any related changes in the surface of the heat-treated HNTs were studied using transmission electron microscopy (TEM, JEOL Ltd, model JEM02100). Impact test, which was a part of mechanical property analysis, was carried out utilizing an izod impact tester (YASUDA SEIKI SEISAKUSHO, Ltd., No. 12353). The impact strength was computed from unnotched specimens hitting it in the middle of the length (following JIS K 7062). The dimension of the specimens is given in Figure 3 whereas the specimen has been shown in Figure 4. agglomeration stat. In Figure 5-(a) the boundaries between the layers are distinctly visible. On the other hand, Figure 5-(b) shows the HNTs with undefined shape. In Figure 5-(c) the HNTs had more uneven surfaces than those in Figure 5-(a). The layed structure of the HNTs has been unified possibly due to dehydration, and thus, the bi-layered hollow tubular shape of HNTs could not be obserced. Past studies showed that the structure of heat-treated HNTs was first transformed from HNT-10Å to HNT-7Å due to loss of interlayer water around 500 C, and then at 700 C, HNT-7Å was completely destroyed to form an amorphous structure, as observed by XRD analysis. Therefore, it can be presumed in our case that the OH groups of HNTs were also eliminated at temperature higher than 500 C via dehydration. Therefore, themperature higher than 500 C had a significant influence on the layer structure of the HNTs and it was anticipated that such structural change should modulate the behavior of the HNTs toward UP. Figure 3 Dimension of impact test specimen (a) Untreated HNTs Figure 4 Specimens for impact test 3. RESULTS AND DISCUSSION 3.1 Structural changes of heat-treated HNTs After heat treatment at 500 C and 700 C, both the untreated and the heat-treated HNTs were examined using TEM, as shown in Figure 5. In all cases, the HNTs exhibited a tubular and layer structure in common. In addition, the HNTs itself formed (b) 500 C heat-treated HNTs
4 (c) 700 C heat-treated HNTs Figure 5 TEM micrographs of UP/HNTs nanocomposites 3.2 Impact strength behaviour of UP/heat-treated HNTs To obtain optimized dispersion conditions for improving mechanical properties, the specimens for impact tests were synthesized using heat-treated HNTs at various temperatures. The intensity of the ultrasound from the ultrasonic homogenizer was kept as a variable and the reinforcement effect of HNT was investigated at various ultrasound intensities. By contrast, for the untreated HNTs no reinforcement effect was observed and the impact stregnth was rather found to be degraded. The 500 C heat-treated HNTs and 700 C heat-treated HNTs nanocomposites showed opposite behaviors. This signified that structural changes of HNTs at various temperature were involved to chemical combination between UP and HNTs. Figure 7 shows the effect of impact reinforcement at 60 W of output power. Comparing the data for untreated HNTs in Figure 6, Figure 7 shows that when the ultrasound intensity was stronger the impact property also improved. Initially, untreated HNT existed in a strongly agglomerated state. It was then dispersed using power greater than the critical stress of untreated HNTs. However, the impact strength of untreated HNT nanocomposites was similar to that of neat UP composites without any nanoparticle. Furthermore, the reinforcing effect of 700 C heattreated HNTs in nanocomposites was excellent and even if the amount of HNTs added to UP the same, suitable output power of ultrasonic homogenizer improved the dispersion of HNTs in UP. Figure 7 Comparison of impact strength under 60 W of ultrasonic homogenization condition Figure 6 Comparison of impact strength under 45 W of ultrasonic homogenization condition The curve in Figure 6 did not reveal any specific trend. Improvement in impact strength was observed only in nanocomposite that contained 700 C heat-treated HNTs, and no other special feature could be identified. In summary the results of impact test were estimated depending on each variable conditions, i.e. ultrasound intensity, heat treatment temperature and content of HNTs. We couldn t find a common pattern between Figure 6 and 7, but under the conditions used in this study the reinforcement effect of HNTs could be partially observed. The nanocomposite with added 700 C heat-treated HNTs was superior with high impact reinforcing effects at 45 W of ultrasound intensity.
5 4. CONCLUSION In this study, the nanocomposites reinforced by heattreated HNTs were synthesized with a view to improving impact property. The primary aim was to develop an optimal synthetic process of nanocomposites with strong bonding between the UP and the HNTs. The HNTs were treated with heat and divided into 3 types; untreated HNTs (at RT), 500 C heat-treated HNTs and 700 C heat-treated HNTs. These HNTs were dispersed by ultrasonic homogenizer at 45 W and 60 W output power of ultrasound intensity. Experimentally, the results analyzed are as follows: (1) A temperature higher than 500 C had a significant influence on the layer structure of the HNTs and caused dehydration between Al-OH (O) layer of HNTs. It was assumed that structural changes of HNTs would cause the HNTs to fuction differently with UP. (2) The toughening effect of HNTs was observed in part; however, more important was preventing aggregations. This can be achieved through optimal dispersion conditions of the HNTs, such as amount ot HNTs in nanocomposite and surface state of HNTs. (3) By controlling the ultrasound intensity when the HNTs were dispersed by ultrasonic homogenizer, some aggregation phenomenon could be improved. (4) The nanocomposite containing 700 C heattreated HNTs was superior with high impact reinforcing effects at 45 W of ultrasound intensity. Hielscher, T. (2005). Ultrasonic production of Nanosize dispersions and emulsions. Dans European Nano Systems Workshop-ENS. Kim, Y. H., Park, S. J., Lee, J. W., & Moon, K. M. (2015). Processing and mechanical properties of nanocomposites based on HNTs and UPR. Modern Physics Letters B, 29, 154003. Wang, Q., Zhang, J., Zheng, Y., & Wang, A. (2014). Adsorption and release of ofloxacin from acid- and heat-treated halloysite.colloids and Surfaces B: Biointerfaces, 113, 51-58. Yuan, P., Tan, D., Faiza, A. B., Yan, W., Fan, W., Liu, M., & He, H. (2012). Changes in structure, morphology, porosity, and surface activity of mesoporous halloysite nanotubes under heating. Clays and Clay Minerals 60(6), 561-573. This work (Grants No. S2328725) was supported by Korean Small Business Innovation Research Program and Research Institute funded Korea Small and Medium Business Administration in 2015. REFERENCES Abdullayev, E., & Lvov, Y. (2010). Clay nanotubes for corrosion inhibitor encapsulation: release control with end stoppers. The Royal Society of Chemistry, 20, 6681-6687. Chiu, H. T., Jeng, R. E., & Chung, J. S. (2004). Thermal cure behavior of unsaturated polyester/phenol blends. Journal of Applied Polymer Science, 91, 1041-1058. Fujii, K., Nakagaito, A. N., Takagi, H., & Yonekura, D. (2013). Sulfuric acid treatment of halloysite nanoclay to improve the mechanical properties of PVA/halloysite transparent composite films. Composite Interfaces, 21(4), 319-327.